NAME

B - The Perl Compiler Backend

SYNOPSIS

use B;

DESCRIPTION

The B module supplies classes which allow a Perl program to delve into its own innards. It is the module used to implement the "backends" of the Perl compiler. Usage of the compiler does not require knowledge of this module: see the O module for the user-visible part. The B module is of use to those who want to write new compiler backends. This documentation assumes that the reader knows a fair amount about perl's internals including such things as SVs, OPs and the internal symbol table and syntax tree of a program.

OVERVIEW

The B module contains a set of utility functions for querying the current state of the Perl interpreter; typically these functions return objects from the B::SV and B::OP classes, or their derived classes. These classes in turn define methods for querying the resulting objects about their own internal state.

Utility Functions

The B module exports a variety of functions: some are simple utility functions, others provide a Perl program with a way to get an initial "handle" on an internal object.

Takes a reference to any Perl value, and turns the referred-to value into an object in the appropriate B::OP-derived or B::SV-derived class. Apart from functions such as main_root, this is the primary way to get an initial "handle" on an internal perl data structure which can then be followed with the other access methods.

The returned object will only be valid as long as the underlying OPs and SVs continue to exist. Do not attempt to use the object after the underlying structures are freed.

amagic_generation

Returns the SV object corresponding to the C variable amagic_generation.

Returns the AV object (i.e. in class B::AV) of the global comppadlist.

regex_padav

Only when perl was compiled with ithreads.

main_cv

Return the (faked) CV corresponding to the main part of the Perl program.

Functions for Examining the Symbol Table

walksymtable(SYMREF, METHOD, RECURSE, PREFIX)

Walk the symbol table starting at SYMREF and call METHOD on each symbol (a B::GV object) visited. When the walk reaches package symbols (such as "Foo::") it invokes RECURSE, passing in the symbol name, and only recurses into the package if that sub returns true.

Functions Returning B::OP objects or for walking op trees

Returns the root op (i.e. an object in the appropriate B::OP-derived class) of the main part of the Perl program.

main_start

Returns the starting op of the main part of the Perl program.

walkoptree(OP, METHOD)

Does a tree-walk of the syntax tree based at OP and calls METHOD on each op it visits. Each node is visited before its children. If walkoptree_debug (see below) has been called to turn debugging on then the method walkoptree_debug is called on each op before METHOD is called.

walkoptree_debug(DEBUG)

Returns the current debugging flag for walkoptree. If the optional DEBUG argument is non-zero, it sets the debugging flag to that. See the description of walkoptree above for what the debugging flag does.

Miscellaneous Utility Functions

ppname(OPNUM)

Return the PP function name (e.g. "pp_add") of op number OPNUM.

hash(STR)

Returns a string in the form "0x..." representing the value of the internal hash function used by perl on string STR.

cast_I32(I)

Casts I to the internal I32 type used by that perl.

minus_c

Does the equivalent of the -c command-line option. Obviously, this is only useful in a BEGIN block or else the flag is set too late.

cstring(STR)

Returns a double-quote-surrounded escaped version of STR which can be used as a string in C source code.

perlstring(STR)

Returns a double-quote-surrounded escaped version of STR which can be used as a string in Perl source code.

class(OBJ)

Returns the class of an object without the part of the classname preceding the first "::". This is used to turn "B::UNOP" into "UNOP" for example.

threadsv_names

In a perl compiled for threads, this returns a list of the special per-thread threadsv variables.

Exported utility variabiles

@optype

my $op_type = $optype[$op_type_num];

A simple mapping of the op type number to its type (like 'COP' or 'BINOP').

@specialsv_name

my $sv_name = $specialsv_name[$sv_index];

Certain SV types are considered 'special'. They're represented by B::SPECIAL and are referred to by a number from the specialsv_list. This array maps that number back to the name of the SV (like 'Nullsv' or '&PL_sv_undef').

OVERVIEW OF CLASSES

The C structures used by Perl's internals to hold SV and OP information (PVIV, AV, HV, ..., OP, SVOP, UNOP, ...) are modelled on a class hierarchy and the B module gives access to them via a true object hierarchy. Structure fields which point to other objects (whether types of SV or types of OP) are represented by the B module as Perl objects of the appropriate class.

The bulk of the B module is the methods for accessing fields of these structures.

Note that all access is read-only. You cannot modify the internals by using this module. Also, note that the B::OP and B::SV objects created by this module are only valid for as long as the underlying objects exist; their creation doesn't increase the reference counts of the underlying objects. Trying to access the fields of a freed object will give incomprehensible results, or worse.

Access methods correspond to the underlying C macros for field access, usually with the leading "class indication" prefix removed (Sv, Av, Hv, ...). The leading prefix is only left in cases where its removal would cause a clash in method name. For example, GvREFCNT stays as-is since its abbreviation would clash with the "superclass" method REFCNT (corresponding to the C function SvREFCNT).

B::SV Methods

REFCNT

FLAGS

object_2svref

Returns a reference to the regular scalar corresponding to this B::SV object. In other words, this method is the inverse operation to the svref_2object() subroutine. This scalar and other data it points at should be considered read-only: modifying them is neither safe nor guaranteed to have a sensible effect.

B::IV Methods

IV

Returns the value of the IV, interpreted as a signed integer. This will be misleading if FLAGS & SVf_IVisUV. Perhaps you want the int_value method instead?

IVX

UVX

int_value

This method returns the value of the IV as an integer. It differs from IV in that it returns the correct value regardless of whether it's stored signed or unsigned.

needs64bits

packiv

B::NV Methods

NV

NVX

B::RV Methods

RV

B::PV Methods

PV

This method is the one you usually want. It constructs a string using the length and offset information in the struct: for ordinary scalars it will return the string that you'd see from Perl, even if it contains null characters.

RV

Same as B::RV::RV, except that it will die() if the PV isn't a reference.

PVX

This method is less often useful. It assumes that the string stored in the struct is null-terminated, and disregards the length information.

It is the appropriate method to use if you need to get the name of a lexical variable from a padname array. Lexical variable names are always stored with a null terminator, and the length field (SvCUR) is overloaded for other purposes and can't be relied on here.

B::PVMG Methods

MAGIC

SvSTASH

B::MAGIC Methods

MOREMAGIC

precomp

Only valid on r-magic, returns the string that generated the regexp.

PRIVATE

TYPE

FLAGS

OBJ

Will die() if called on r-magic.

PTR

REGEX

Only valid on r-magic, returns the integer value of the REGEX stored in the MAGIC.

B::PVLV Methods

TARGOFF

TARGLEN

TYPE

TARG

B::BM Methods

USEFUL

PREVIOUS

RARE

TABLE

B::GV Methods

is_empty

This method returns TRUE if the GP field of the GV is NULL.

NAME

SAFENAME

This method returns the name of the glob, but if the first character of the name is a control character, then it converts it to ^X first, so that *^G would return "^G" rather than "\cG".

It's useful if you want to print out the name of a variable. If you restrict yourself to globs which exist at compile-time then the result ought to be unambiguous, because code like ${"^G"} = 1 is compiled as two ops - a constant string and a dereference (rv2gv) - so that the glob is created at runtime.

If you're working with globs at runtime, and need to disambiguate *^G from *{"^G"}, then you should use the raw NAME method.

STASH

SV

IO

FORM

AV

HV

EGV

CV

CVGEN

LINE

FILE

FILEGV

GvREFCNT

FLAGS

B::IO Methods

LINES

PAGE

PAGE_LEN

LINES_LEFT

TOP_NAME

TOP_GV

FMT_NAME

FMT_GV

BOTTOM_NAME

BOTTOM_GV

SUBPROCESS

IoTYPE

IoFLAGS

IsSTD

Takes one arguments ( 'stdin' | 'stdout' | 'stderr' ) and returns true if the IoIFP of the object is equal to the handle whose name was passed as argument ( i.e. $io->IsSTD('stderr') is true if IoIFP($io) == PerlIO_stdin() ).

B::AV Methods

FILL

MAX

ARRAY

ARRAYelt

Like ARRAY, but takes an index as an argument to get only one element, rather than a list of all of them.

OFF

This method is deprecated if running under Perl 5.8, and is no longer present if running under Perl 5.9

AvFLAGS

This method returns the AV specific flags. In Perl 5.9 these are now stored in with the main SV flags, so this method is no longer present.

B::CV Methods

STASH

START

ROOT

GV

FILE

DEPTH

PADLIST

OUTSIDE

OUTSIDE_SEQ

XSUB

XSUBANY

For constant subroutines, returns the constant SV returned by the subroutine.

CvFLAGS

const_sv

B::HV Methods

FILL

MAX

KEYS

RITER

NAME

ARRAY

PMROOT

This method is not present if running under Perl 5.9, as the PMROOT information is no longer stored directly in the hash.